In the planning and implementation of wind parks, the levels of sound emission to be expected are playing an increasingly important part in terms of approval and acceptance. The sound immission calculations which are required for that purpose are generally based on measured sound output levels (LW) of the wind power installations and are implemented with current software programs. Sound immission measurements are also taking place more and more frequently so that the sound level of an individual installation or the sound level of an entire wind park can be ascertained at a given immission point (outside or within the wind park). If the immission point is an individual house, a housing estate or the like, there a certain sound output level must be observed at least over given periods of time, for example between 10 pm and 6 am, in accordance with the statutory provisions and the provisions relating to planning policy. It is also not permitted for the sound output level at an immission point, for example a housing estate, to rise beyond all measure during the day.
The interest in observing the required sound output levels at one or more immission points is in conflict with the interest on the part of the operator of the wind park of achieving the maximum energy yield with the wind park. Admittedly, by means of total shutdown of all wind power installations of the wind park at night, the observance of sound limit values would be satisfied, but the entire wind park operation would then be highly ineffective.
Therefore the object of the present invention, in spite of observing sound output limit values at one or more immission points, is to maximize the energy yield of the wind park according to the respective time of day, wind direction and wind strength.
The invention is based on the realization that the sound emission of a wind power installation is dependent on various parameters and categories. When considered in rough terms, sound production can be subdivided into two categories. One category is tone relatedness, that is to say the generation of sound by the machine part (drive part) of the wind power installation. That is also identified as so-called mechanical sound. The other category is a wide-band noise of the wind power installation, caused by aerodynamic sound by the rotor blades and other wide-band machine noises, for example fans.
The so-called mechanical sound can originate from various mechanical components such as for example a generator and/or a transmission assembly. The sound is transmitted for example by way of the structure of the installation and is radiated from the surfaces thereof. That sound is generally of tone-related character, that is to say, sound irradiation can be association with a given frequency. The sound sources can be determined so that it is possible to take suitable countermeasures (for example decoupling in respect of sound conducted through solids).
The sound output level of a wind power installation is however substantially determined by the aerodynamic noises which are produced by the air flows at the rotor blades. As sound generation is related approximately at the fifth power to the blade tip speed, an increased rotary speed always also entails an increased level of sound emission. That noise involves a wide-band frequency spectrum. For a rotor diameter of 40 m, a change in the sound output level by 1 dB(A) was ascertained, per rotor revolution per minute. It is to be noted in that respect that a reduction in the sound output level by 3 dB(A) already corresponds to a halving of that value (in acoustically audible terms).